Method for producing a film by cu2znsns4 silar

ABSTRACT

The present invention relates to a method for manufacturing a film of Cu 2 ZnSnS 4  (CZTS) on a metal or metal oxide substrate by successive ion layer adsorption and reaction (SILAR), characterized by the fact that it comprises at least two cycles, each cycle comprising the following successive steps: immersion of the substrate in a cationic solution comprising copper sulfate (CuSO 4 ), tin sulfate (SnSO 4 ) and zinc sulfate (ZnSO 4 ), referred to as the precursors; rinsing by immersion of the substrate in deionized water; immersion of the substrate in an anionic solution comprising sodium sulfide (Na 2 S); rinsing by immersion of the substrate in deionized water; the concentration ratio of the copper (Cu 2+ ), zinc (Zn 2+ ) and tin (Sn 2+ ) cations and of the sulfur (S 2− ) anion is 1:1:1:1 or 1:0.5:0.5:1. 
     The invention will be applicable more particularly in the photovoltaic field, especially for production of thin-film, especially nanostructured photovoltaic cells, such as the ETA (Extremely Thin Absorber) cell.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a film of Cu₂ZnSnS₄, also referred to as CZTS, by adsorption and reaction of successive ionic layers (SILAR in English: Successive Ion Layer Adsorption and Reaction).

This material based on abundant and non-toxic elements has the properties of a semiconductor with a band gap of approximately 1.5 eV. This material is very advantageous for applications in the photovoltaic range, especially for the production of thin-film photovoltaic cells or in particular of nanostructured cells such as the ETA (Extremely Thin Absorber) cell.

PRIOR ART

Chemical or electrochemical deposition techniques, such as electrodeposition, spray pyrolysis or chemical bath, have been used. However, they have not made it possible to achieve uniform films of controlled thickness.

CZTS also is synthesized from chemical reactions in the solid state between zinc sulfide (ZnS), copper sulfide (Cu₂S) and tin sulfide (SnS₂). However, impurities such as ternary and secondary compounds form more readily than CZTS, and it is therefore difficult to obtain only CZTS.

The SILAR method, in which a substrate is immersed separately in two solutions of precursors and rinsed with water between each immersion, is also known. A SILAR cycle consists in the adsorption of cationic precursors then the adsorption of anionic precursors, with an intermediate rinsing step. The adsorbed precursors will react together to form the CZTS. This method has been investigated by Shinde et al. (Room temperature novel chemical synthesis of Cu2ZnSnS4 (CZTS) absorbing layer for photovoltaic application. Materials Research Bulletin.) using a cationic solution of 0.1 M copper chloride (CuCl), 0.05 M zinc sulfate (ZnSO₄) and 0.05 tin chloride (SnCl₄) and an anionic solution of 0.2 M thioacetamide (C₂H₅NS). The CZTS obtained is rich in copper and non-stoichiometric. In addition, impurities in the form of CTS (Cu₂SnS₃) are also produced. This cationic solution has an acid pH of 3, which greatly limits the type of substrate than can be used; in particular, it is capable of dissolving the metal oxides.

A need therefore exists to propose a method that remedies all or part of these disadvantages.

DESCRIPTION OF THE INVENTION

For this purpose the present invention relates to a method for manufacturing a film of CZTS on a metal or metal oxide substrate by SILAR.

According to one aspect of the invention, the cationic solution comprises precursors in the form of copper sulfate (CuSO₄), tin sulfate (SnSO₄) and zinc sulfate (ZnSO₄).

The choice of these precursors makes it possible to obtain a cationic solution in which the pH, although still acidic, is 5. At this pH, the cationic solution can be used on metal or metal oxide substrates without the risk of damaging them.

Preferentially, the Cu²⁺, Zn²⁺ and Sn²⁺ cations are used in a concentration ratio of 1:0.5:0.5, or preferably of 1:1:1. With these ratios, the formation of the CZTS takes place as a film without impurity.

Advantageously, the anionic solution comprises sodium sulfide (Na₂S). The ratio with the S²⁻ anion is 1:0.5:0.5:1, or else 1:1:1:1.

According to one aspect of the invention, the precursors of the cationic and anionic solutions have a very low concentration on the order of 5 to 20 mM.

Other objectives and advantages will become apparent in the course of the description hereinafter, which presents a detailed embodiment of the invention but is not to be construed as limitative thereof.

Beforehand, different advantageous but non-limitative embodiments of the invention that may be used alternatively or cumulatively are introduced.

As a reminder, the present invention relates to a method for manufacturing a film of Cu₂ZnSnS₄ (CZTS) on a metal or metal oxide substrate by successive ion layer adsorption and reaction (SILAR), characterized by the fact that it comprises at least two cycles, each cycle comprising the following successive steps: immersion of the substrate in a cationic solution comprising copper sulfate (CuSO₄), tin sulfate (SnSO₄) and zinc sulfate (ZnSO₄), referred to as the precursors; rinsing by immersion of the substrate in deionized water; immersion of the substrate in an anionic solution comprising sodium sulfide (Na₂S); rinsing by immersion of the substrate in deionized water; the concentration ratio of the copper (Cu²⁺), zinc (Zn²⁺) and tin (Sn²⁺) cations and of the sulfur anion (S²⁻) anion is 1:1:1:1 or 1:0.5:0.5:1.

According to additional or alternative variants, the method is such that:

-   -   the concentration of each precursor in the cationic solution is         between 5 and 20 mM;     -   the concentration of sodium sulfide (Na₂S) in the anionic         solution is between 5 and 20 mM;     -   the concentrations of the precursors in the cationic solution         are 10 mM copper sulfate, 10 mM tin sulfate and 10 mM zinc         sulfate;     -   in the anionic solution, the concentration of sodium sulfide is         10 mM;     -   the duration of immersion of the substrate in the cationic or         anionic solution is between 15 and 25 seconds;     -   for rinsing, the duration of immersion of the substrate,         advantageously in the deionized water, is longer than or equal         to 15 seconds.     -   the cationic solution and/or the anionic solution is an aqueous         solution;     -   the cationic solution has a pH of 5;     -   the anionic solution has a pH of 12;     -   after the at least two cycles, it comprises a step of annealing,         preferentially at 400° C., advantageously under a neutral         atmosphere, for example under vacuum or under a stream of argon;     -   the substrate is nanostructured;     -   the substrate contains nanowires of zinc oxide;     -   it comprises at least fifteen cycles, preferentially from         fifteen to twenty.

According to another aspect, the invention relates to a nanostructured surface covered by a homogeneous film having a minimum thickness of 10 nm of a CZTS phase obtained by the method such as described in the foregoing.

According to another aspect, the invention relates to nanowires of metal oxide covered by a homogeneous film of a CZTS phase obtained by the method such as described in the foregoing.

DETAILED DESCRIPTION OF THE INVENTION

The present method permits the formation of a film of Cu₂ZnSnS₄, also referred to as CZTS. By film, there is understood a thin skin of a material that continuously covers a surface. Advantageously, the film is composed of a Cu₂ZnSnS₄ phase.

One of the advantages of the method according to the invention is that it permits the formation of a film of Cu₂ZnSnS₄ on a metal or metal oxide substrate. Advantageously, the substrate is nanostructured. By nanostructured, it is understood that the substrate has at least one dimension varying between 1 and about 100 nanometers. By way of a preferred example, the substrate has the form of nanowires. A nanowire is a wire in which the diameter does not exceed 500 nm.

The substrate comprises, for example, zinc oxide (ZnO), titanium oxide (TiO2), indium tin oxide (ITO), tin oxide (SnO2), aluminum-doped zinc oxide (AZO), CuSCN, steel, molybdenum (Mo), copper (Cu), cadmium sulfide (CdS) and zinc sulfide (ZnS).

The manufacturing method is based on SILAR technology. The method therefore comprises the successive immersion of the substrate in a cationic solution, then in an anionic solution, with intermediate rinsing and final rinsing.

According to the invention, all of the steps of a cycle are performed at a temperature between 18 and 25° C.

By immersion, it is understood that the substrate is dipped in a bath.

These four immersions together constitute one cycle. The method according to the invention comprises at least two cycles, preferentially at least fifteen cycles, preferentially from fifteen to twenty cycles. The number of cycles depends on the desired film thickness. Between fifteen and twenty cycles, the film thickness is sufficient to form a continuous and homogeneous film. For example, a film having a thickness of 20 nm is obtained in fifteen cycles.

By homogeneous, it is understood that the film comprises a single phase of CZTS. Advantageously, the CTS phase is less than 5%.

Advantageously, therefore, the film is composed only of a CZTS phase. According to another embodiment, the film is composed of a CZTS phase and of a CST phase in a proportion smaller than 5%.

The film thickness is regular. By way of example, the thickness is from at least 10 nm to more than 500 nm with a variation on the order of 5 nm. By way of example, a thickness of 20-40 nm will be sought for an ETA cell and a thickness greater than 500 nm for a thin-film cell. The cycles are repeated until the desired thickness is obtained. Preferentially, the solutions are renewed beyond fifteen to twenty cycles, to maintain a constant concentration of the species and a constant rate of reaction.

According to a preferred embodiment, the cationic solution comprises precursors of the CZTS. These precursors present in the cationic solution are metal sulfates of the CZTS. More precisely, the cationic solution comprises copper sulfate (CuSO₄), zinc sulfate (ZnSO₄) and tin sulfate (SnSO₄). The cations in solution in the cationic solution are the zinc (Zn²⁺), tin (Sn²⁺) and copper (Cu²⁺) cations.

The choice of metal precursors of sulfate type contributes to obtaining a cationic solution in which the pH is acid and compatible with metal oxides. Preferably, the pH is on the order of 5.

Particular study has been devoted to the concentrations of the cations of the precursors in the present method. Preferentially, the concentrations are low; advantageously a cationic solution comprises between 5 and 20 mM of each precursor.

Surprisingly, the method according to the invention forms a satisfactory film of CZTS for ratios of the Cu²⁺, Sn²⁺ and Zn²⁺ cations respectively of 1:0.5:0.5 or preferably 1:1:1. It is preferred that the tin and zinc cations are in equivalent concentrations. The concentration of the copper cation is either equivalent or two times higher. These ratios are very advantageous.

Only a ratio of 1:1:1 or 1:0.5:0.5 makes it possible to obtain a homogeneous film with a single CZTS phase.

The anionic solution advantageously comprises sodium sulfide (Na₂S). The concentration of sodium sulfide in the anionic solution is advantageously quite low, preferentially between 5 mM and 20 mM. The anionic solution is advantageously composed of sodium sulfide (Na₂S) diluted in water.

According to the invention, the concentrations of the precursors and of sodium sulfide are chosen so that the concentration ratio of the Cu²⁺, Sn²⁺ and Zn²⁺ cations and of the S²⁻ anion is 1:0.5:0.5:1 or else 1:1:1:1.

The formation of CZTS does not depend linearly on the concentrations of the cationic and anionic species. In particular, the existence of a majority CTS phase in the prior art is not resolved by an excess of zinc relative to tin; to the contrary, the CTS phase is reinforced.

The solution (Na₂S) is very stable in time. The solution pH is on the order of 12. The elevated pH of the anionic solution is not harmful for the substrate. The sodium sulfide does not attack the surface of the substrate, because the S²⁻ anions are adsorbed on the cationic layer formed during the immersion of the substrate in the cationic solution. The cationic layer functions to protect the substrate.

The cationic and anionic solutions are advantageously aqueous solutions, wherein the cationic precursors and the sodium sulfide are dissolved in deionized water.

To obtain a continuous and homogeneous film, the time of immersion of the substrate in the solutions is a parameter which was particularly studied.

The duration of immersion of the substrate in the cationic solution is advantageously between 15 and 25 seconds. For obtaining a continuous and homogeneous cationic layer, it is not necessary to leave the substrate immersed for a long time. In this way the formation of excessively large particles generating large relief, is avoided. Preferably, nanoparticles having a diameter on the order of 5 nm are formed.

The rinsing steps, in which the substrate is immersed in a bath, advantageously of deionized water, have an important function for the formation of the continuous and homogeneous film. The rinsing is intended to remove the excess species.

The duration of immersion is therefore controlled. Advantageously it lasts at least 15 seconds, preferentially 20 seconds.

Scanning electron microscopy (SEM) or scanning transmission electron microscopy (STEM) is used to characterize the surface of the substrate and to follow the formation of a film of CZTS.

In the images hereinafter, it is possible to see nanowires of ZnO covered with Cu₂ZnSnS₄. The technique of x-ray diffraction is not specific enough to detect the CZTS phase, because the diffraction peaks of the CZTS are very close to and even coincide with those of the secondary and tertiary phases. The Raman technique is preferred for differentiating the secondary and tertiary phases of the quaternary phase of CZTS.

EXAMPLES

The method according to the invention was used to form a deposit of CZTS by the SILAR technique on ZnO nanowires. The duration of immersion in the cationic then anionic solutions was 20 seconds each, and the duration of immersion for rinsing in a bath of deionized water was 20 seconds

TABLE 1 Description of the different methods used for deposition of CZTS by the SILAR technique. Anionic Method Cationic solution solution P2  5 mM CuSO₄  5 mM SnSO₄ 10 mM ZnSO₄ 10 mM Na₂S P3 10 mM CuSO₄  5 mM SnSO₄ 10 mM ZnSO₄ 10 mM Na₂S P4 10 mM CuSO₄  5 mM SnSO₄  5 mM ZnSO₄ 10 mM Na₂S P5 10 mM CuSO₄ 10 mM SnSO₄ 10 mM ZnSO₄ 10 mM Na₂S

FIG. 1: SEM image from above of Cu₂ZnSnS₄ deposited by SILAR on ZnO using method P5.

FIGS. 2 and 3: STEM image of a structure comprising a ZnO nanowire core and Cu₂ZnSnS₄ shell using method P5.

Analyses by the Raman technique were carried out on the CZTS films obtained by the methods of Table 1.

FIGS. 4 and 5

These two spectra correspond to the CZTS films obtained respectively by methods P4 and P5 of Table 1 without annealing step and after annealing at 400° C. in a stream of argon for 1 h.

A majority phase of CZTS is observed.

With method P4, a majority phase of CZTS of at least 95% is obtained.

With method P5, a single phase of CZTS comprising 100% CZTS is obtained. The peak is less intense, because the film must be slightly thinner and may be less crystallized.

FIG. 6

This spectrum corresponds to the films of CZTS obtained respectively by methods P2, P3, P4 and P5 of Table 1 after a step of annealing at 400° C. under a stream of argon. A majority phase of CZTS is observed for P4 and P5, whereas the majority phase for P2 and P3 is CTS. 

1. A method for manufacturing a film of Cu₂ZnSnS₄ (CZTS) on a metal or metal oxide substrate by successive ion layer adsorption and reaction (SILAR), wherein the method comprises at least two cycles, each cycle comprising, in the following order: immersing the substrate in a cationic solution comprising copper sulfate (CuSO₄), tin sulfate (SnSO₄) and zinc sulfate (ZnSO₄), as precursors; rinsing, by immersion, the substrate in deionized water; immersing the substrate in an anionic solution comprising sodium sulfide (Na₂S); and rinsing, by immersion, the substrate in deionized water; wherein a concentration ratio of copper (Cu²⁺), zinc (Zn²⁺) and tin (Sn²⁺) cations and of sulfur anion (S²⁻) anion is 1:1:1:1 or 1:0.5:0.5:1.
 2. The method according to claim 1, wherein a concentration of each precursor in the cationic solution is between 5 and 20 mM.
 3. The method according to claim 1, wherein a concentration of sodium sulfide (Na₂S) in the anionic solution is between 5 and 20 mM.
 4. The method according to claim 1, wherein the concentrations of the precursors in the cationic solution are 10 mM copper sulfate (CuSO₄), 10 mM tin sulfate (SnSO₄) and 10 mM zinc sulfate (ZnSO₄).
 5. The method according to claim 1, wherein a concentration of sodium sulfide (Na₂S) in the anionic solution is 10 mM.
 6. The method according to claim 1, wherein a duration of immersion of the substrate in the cationic or anionic solution is between 15 and 25 seconds.
 7. The method according to claim 1, wherein a duration of immersion of the substrate in the deionized water for rinsing is longer than or equal to 15 seconds.
 8. The method according to claim 1, wherein the cationic solution, the anionic solution, or both, is an aqueous solution.
 9. The method according to claim 1, wherein the cationic solution has a pH of
 5. 10. The method according to claim 1, wherein the anionic solution has a pH of
 12. 11. The method according to claim 1, further comprising, after the at least two cycles, annealing at 400° C. under a neutral atmosphere.
 12. The method according to claim 1, wherein the substrate is nanostructured.
 13. The method according to claim 1, wherein the substrate comprises nanowires of zinc oxide.
 14. The method according to claim 1, comprising at least fifteen cycles.
 15. A nanostructured surface covered by a homogeneous film having a minimum thickness of 10 nm of a CZTS phase obtained by the method according to claim
 1. 16. A nanowire of metal oxide covered by a homogeneous film of a CZTS phase obtained by the method according to claim
 1. 17. The method according to claim 1, comprising from fifteen to twenty cycles. 